The present invention relates generally to chromium electroplating and more particularly to an improved method of electrolytically depositing chromium on a previously chromium-plated surface.
Chromium-plated articles are widely used throughout industry where a hard, chemically resistant metal surface is required. Such articles are prepared using conventional chromium electroplating techniques well known in the art. There are various instances where it is desirable or even necessary to electroplate additional chromium onto a previously chromium-plated article. However, it is well recognized in the industry that it is very difficult to successfully plate chromium onto an article having a previously deposited chromium surface. This problem, generally known as the "chrome restart problem," is known to arise even when an attempt is made to deposit an additional layer of chromium upon a relatively fresh chromium-plated surface only a few hours old. Specifically, the layer of new or additional chromium does not adhere well to the underlying chromium layer, with the result that delamination or blistering often occurs along the interface between the new and the old chromium layers.
It is also known that it is often difficult to successfully initiate chromium electrodeposition on certain other meatl surfaces, particularly iron and stainless steel. A method known as "nickel striking" is sometimes used to initiate deposition of chromium onto these metals. According to this method, a thin layer of nickel is electroplated onto, for example, a stainless steel surface from a nickel "strike," or plating solution. Nickel electrodeposition onto stainless steel is more readily accomplished than is chromium electrodeposition, and the resulting thin layer of nickel is relatively receptive to subsequent chromium electroplating. The nickel layer thus forms a base on the stainless steel that helps to ensure successful subsequent chromium plating. This method, although moderately successful, is necessarily somewhat time-consuming and requires maintenance of separate nickel and chromium plating baths.
The difficulty of depositing an additional layer of chromium onto a preexisting chromium-plated surface also makes it difficult to form a chromium layer of appreciable thickness on an article. As in the electrolytic deposition of many metals, chromium is not deposited in a perfectly smooth layer, but rather tends to form nodules and "trees" (small crystalline outgrowths) of chromium on the article as the electroplating process is carried on over a long period of time. Accordingly, to form a chromium-plated article having an appreciable thickness of chromium it has been necessary to deposit a single chromium layer in considerable excess and thereafter mill or grind the article to obtain a smooth, uniform layer of chromium of desired thickness. This method is time-consuming and relatively inefficient, particularly since the plating process typically takes a day or more and cannot be interrupted to measure the thickness of accumulated chromium without risk of being unable to restart the plating process. Consequently, the process is customarily carried out continuously for a long period of time to ensure that a sufficient thickness of chromium is deposited to allow milling to the desired thickness at all points. Nevertheless, there is an occasional failure of the electrodeposition to initiate properly. Since the article cannot be inspected during the plating process, such a failure goes undetected for a day or more until the entire process is completed. At that time the only recourse in the event of failure is to strip the chromium and repeat the process.
The chrome restart problem is also particularly acute where chromium-plated metal articles must be maintained within very precise dimensional tolerances in applications where the chromium plating is subject to wear and abrasion, as in the case for example of chromium-plated steel bearings. It is desirable in such applications to be able to deposit later an additional hard chromium layer as the initial chromium surface becomes worn. Until now it has been difficult or impossible to maintain a sufficiently thick chromium layer because of the difficulty of successfully restarting the chromium plating process. Instead of even attempting to plate additional chromium, the usual approach has been to completely strip the remaining original chromium and replate the article. Such stripping is time-consuming and is also undesirable because the use of corrosive chemical stripping agents involves a risk of damaging the underlying metal article. Also, the subsequent replating is subject to the problems mentioned above.
Accordingly, it is the general object of the present invention to provide a method of electroplating chromium that is mre reliable and efficient than has heretofore been available.
It is also an object of the present invention to provide a method of electroplating chromium onto the surface of a previously chromium-plated article.
It is another object of the present invention to provide a method for treating metal articles in preparation for chromium electroplating thereon.
It is yet another object of the present invention to provide an efficient and reliable method of electroplating a relatively thick layer of chromium onto a metal article.
It is a more specific object of the present invention to provide a method of treating a previously chromium-plated surface in preparation for subsequent electroplating of an additional chromium layer onto said surface.